The technical solution itself is not a big issue - it's not hard to host a file or to make an Electron API call. What's difficult (and more and more so) is to maintain all these applications and infrastructure as they keep growing in complexity.
Here we are talking about updating the infrastructure to maintain and update dictionaries (presumably one per language) and to make new Electron API calls (and those get breaking changes very often). Should we actually do that? Is it that crucial to provide that level of control for each http request? I think that's something that should be discussed because it's apparently assumed that we have some moral obligation or something to provide that control, but I don't think we do. We have to be pragmatic and weight each solution against the maintenance cost.
Here the value is low for most users (many don't care, and those who do already have Little Snitch or similar installed), and the maintenance cost is relatively high. So because of this I'm not convinced it's worth supporting this feature.